If there weren't already a film called World's Greatest Dad, that over-commodified Father's Day slogan would have made a fine title for Matt Ross' Captain Fantastic, which stars Viggo Mortensen as one of the screen's all-time most progressive patriarchs — the surviving half of a counter-culture couple who decided to shrug off capitalism (they ignore Christmas, but celebrate "Noam Chomsky Day"), quit the big city and raise their family off the grid. But when Mom commits suicide, that puts a major twist in the fairy tale, which is precisely where Ross' emotionally gripping family drama begins, evenhandedly weighing the pros and cons of its wildly unconventional parenting strategy, as the grieving father and his six kids cope with the idea of integrating back into polite society.

Boasting half a dozen impressive youth performances alongside a leading role that takes full advantage of Mortensen's own sensitive, back-to-nature spirit, Captain Fantastic easily ranks among the most polished and relatable of this year's Sundance offerings, even if the self-sufficient lifestyle it depicts will feel totally alien to most of its mainstream audience. Though it's been just four years since Ross' debut, 28 Hotel Rooms, premiered at Sundance, his writing and directing skills have progressed so much in that time, this sophomore effort could pass for a fourth or fifth feature, boasting studio-caliber production values while preserving the personal ambition of an independently made passion project.

Opening somewhere so far removed from civilization that all we see are trees, the film plunges into the primordial Washington forest where Ben (Mortensen, in full mountain-man beard) has organized a rite-of-passage hunting ceremony for his eldest son, Bodevan (23-year-old British actor George MacKay, a slightly alien but appropriately youthful-looking choice). There's an almost cult-like quality to this ritual, which stands to reason: What are families, after all, beyond autonomous little sects forced to operate within a broader social context? Only in Ben's case, he's effectively cut the ties that connect the family to the rest of the world — so much so that it's not until days later, when reunited with the nearest telephone line, that he learns what's happened to his wife, Leslie.

"Your mother is dead," Ben matter-of-factly announces, speaking to his children with a directness American parents seldom display toward minors. This is Ben's way: He treats his kids as young adults, respecting their intelligence at every turn — as demonstrated in a visit to their more conventionally minded aunt and uncle (Kathryn Hahn and Steve Zahn, respectively), whose relatively average sons are all but glued to their iPhones. Though Ben's children know nothing of Lady Gaga or "Star Trek," as a result of rigorous home-schooling, they are conversant in everything from the Constitution to Karl Marx — not to mention wilderness survival, self defense, extreme rock climbing and human reproduction.

The expression on youngest son Nai's face is priceless in a scene in which the kids all receive presents. His siblings each get hunting knives, while the 6-year-old receives a copy of "The Joy of Sex." It's a telling moment, topped only by an impromptu quiz for the daughter who's decided to skip ahead in her reading and try "Lolita" — which she analyzes with the insight of a college student, or a professional critic.

On one hand, Ben has clearly raised his children to be respectful, bright young adults, but he's also guilty of sheltering them in his own way, withholding how to interact with others when and if they ever decide to leave the bohemian forest life. It's as if he hasn't factored actual adulthood into his parenting plan, giving them each names as unique as their respective intellects, with the socially awkward result that they all sound like characters from "The Lord of the Rings": In addition to Bodevan and Nai (Charlie Shotwell), there's rebellious son Rellian (Nicholas Hamilton), towhead Zaja (Shree Crooks) and beautiful teen daughters Kielyr (Samantha Isler) and Vespyr (Annalise Basso) — each given moments in which their individual personalities can shine and further imbued with distinct qualities by the actors who play them.

With Leslie's passing, the moment for the family to confront the real world comes sooner than Ben could have imagined, causing all sorts of uncomfortable moments. The challenge to assimilate is most pronounced for Bodevan, who betrays his father by discreetly applying to a handful of colleges (at his mother's encouragement). When the young man meets a pretty blonde teen at an overnight campsite, he recognizes the rising emotions from books, but badly embarrasses himself by proposing marriage immediately following their first kiss.

Captain Fantastic is full of such well-chosen situations, illustrating both the texture of their life before (further realized by a wardrobe of secondhand and hippie clothes, the work of Quentin Tarantino's costume designer Courtney Hoffman) and the shock of mixing with "normal" folks (as in a hilarious scene in which the body-shameless Mortensen steps off the family bus in the buff). It's clear that Ross has not only done his research, but put his imagination to work, more often than not dramatizing — as opposed to directly articulating — the underlying themes.

The glaring exception to that principle arrives in the form of Leslie's father, Jack (sternly played by Frank Langella), whose domineering manner overshadows his relatively understanding wife (Ann Dowd). Whether parent or child, practically anyone can relate to the tensions represented in Captain Fantastic: It can be hard to resist the urge to decide how someone else raises their kids — which is precisely Jack's problem, though in many respects, he does have a point, even if the character himself feels like something of a cardboard villain.

In addition to wanting the best for his grandkids, Jack is determined to give his daughter a proper funeral. In doing so, he's not only ignoring Leslie's last wishes, but exerting one last bout of control over her free-spirited nature, effectively forcing Ben to drive his bus all the way to New Mexico for the confrontation he'd been hoping to avoid. The inspired choice of casting Mortensen — a natural Papa Bear, who taps into both his physical strength and spiritual gentleness — shows through best when interacting with the kids, though the actor also shines when forced to defend his choices to others.

Trusting the integrity of the situation (far better dramatized than in Cedric Kahn's similarly themed Wild Life), Ross doesn't run from the resulting sentimentality the way so many other directors do; nor does he undercut it with irony or sarcasm as has become the regrettable tendency in independent cinema. Instead, he respects the emotions of both his characters and audience throughout, and though that means risking derision from cynics, Captain Fantastic should connect in a major way with those looking to be swept up and moved by such a fundamental human experience. The fact that it looks and sounds so great in the process — Ross scored a major coup in hiring Jacques Audiard's d.p. Stephane Fontaine (A Prophet, Rust & Bone) — makes for an experience that feels as luminous and enriching as practically every widescreen frame.